Now, let’s look at
the placenta itself.
We saw in the earlier diagrams how the villi
are becoming more complex. This is a view
in time with a time dimension along the axis,
and it shows the increasing complexity of
the villi themselves projecting into the syncytiotrophoblast
with maternal blood flowing through the syncytiotrophoblast.
Eventually, embryonic blood vessels will develop
inside the villi. But you can see the embryonic
blood and maternal blood never actually come
in direct contact with each other. It always has
to cross a membrane composed of the trophoblast
cells and the syncytiotrophoblast cells in
order to exchange nutrients and gasses between
the two of them.
Again, here’s a picture which we looked
at previously, which just shows you what the
illustration might look like in real life.
The diagram is very helpful from the point
of view of labeling, but the photograph gives
you an idea of what it would actually look like.
Now, what’s the placenta doing? There are
five major functions which are being fulfilled
by the placenta. Some of which we’ve mentioned
already. So, one of its major purposes is
the gas exchange. Oxygen is coming from the
mother and being taken up by the embryonic
and fetal blood. That requires that that embryonic
and fetal blood has to have a higher affinity
for oxygen than normal adult human blood so
that the developing embryo or fetus can extract
oxygen from the maternal blood across that
membrane that we discussed previously.
Equally, waste gasses will diffuse back across
there and into the maternal blood and be dealt
with by the mother’s respiratory system
after that. It’s not just gasses that are
involved, but also nutrients and waste products.
So nutrients being supplied by the mother
to the developing embryo and fetus and waste
products being removed from the embryo and
fetus into the mother’s blood, and then
dealt with by the mother’s own systems subsequently.
And of course, if there are to be some adverse
environmental factor present, then it may
be able to pass the placenta. We know, for
instance, that smoking during a course of
pregnancy, will significantly decrease the
size of the placenta and make it much more
likely that things will go wrong during the
course of the pregnancy. However, that’s
not the only functions fulfilled by the placenta.
It’s also a major site of hormonal action
right from very early times so that one of
the early signals that pregnancy has occurred
is the release of human chorionic gonadotropin
into the maternal blood. And that’s the
basis of many pregnancy tests. That is mediated
by the tissues that will become placenta
Then finally, the embryo is itself different.
It is genetically different from the mother.
So, why is it not rejected in the way that
the mother would reject an organ transplant
unless she was given drugs to suppress her
immune system? What is it that protects the
embryo and fetus from being rejected as foreign
tissue? The answer is that we still have much
to learn about this. But there are aspects
of it that we know that are mediated by the
placenta itself. So it seems to create an
immunologically privileged site which allows
the developing embryo and fetus not to be
rejected. Of course, if we knew how that worked,
that might give us clues for dampening down
rejection of tissues in organ transplants.
Therefore, that’s a major site of research
interest at the moment.
This is a picture of a placenta seen after
birth. The baby is born first and then the
placenta is delivered afterward, as I’m
sure you know. This is a slightly unusual
one and that the umbilical cord is at the
edge. But that’s not a variant that would
cause any difficulty or harm during the pregnancy.
So, what we looked at in this particular lecture?
We’ve looked at folding of the embryo. We’ve
looked at the development of the extra-embryonic
cavities focusing on the amniotic cavity,
and the membranes that surround them. And
then we closed by looking at the development
and the functions carried out by the placenta.
Thank you very much.